FIG. 8 is a schematic view of the reaction tube of a prior art apparatus which is used for metal organic chemical vapor deposition (hereinafter referred to as "MOCVD"). In FIG. 8, reference numeral 2 designates a reaction tube made of quartz having a gas inlet 1 at the top portion thereof. An exhaust gas outlet 3 is provided at the bottom of reaction tube 2. Coils 4 for heating a carbon susceptor 5 with high frequency electromagnetic waves are disposed outside and surrounding the reaction tube 2. The carbon susceptor 5 is disposed inside the reaction tube 2. A wafer 6 is disposed on the susceptor 5.
A method of growing an AlGaAs epitaxy layer utilizing this prior art apparatus will be described.
When an AlGaAs layer is epitaxially grown, GaAs is generally used as a substrate, and trimethyl-gallium (TMGa), trimethyl-aluminum (TMAl), and arsine (AsH.sub.3) are used as the source gases. Although there are various kinds of doping gases, hydrogen selenide (H.sub.2 Se) is generally used as a donor source and dimethyl-zinc (DMZn) or diethyl zinc (DEZn) is generally used as an acceptor source. These gases are introduced into the reaction tube 2 from the gas inlet 1 together with the hydrogen as a carrier gas. The gases introduced into the reaction tube 2 are decomposed at places on or near the GaAs substrate 6. An AlGaAs epitaxial layer is grown on the GaAs substrate 6 on the basis of the following reaction. EQU xTMAL+(1-x)TMGa+AsH.sub.3 .fwdarw.Al.sub.x Ga.sub.1-x As+3CH.sub.4
The compisition of the mixed crystal film grown can be controlled by varying the composition ratio of source gases. The conductivity type and the carrier concentration of epitaxial layer can be controlled by mixing doping gases into the source gases. Since the above-described doping gases such as H.sub.2 Se, DMZn, or DEZn have quite a high decomposition speed at the growth temperature e.g., 600.degree. to 750.degree. C., employed in the ordinary MOCVD method, the carrier concentration of the doped epitaxy layer is regulated by the dopant supply rate. The carrier concentration of epitaxial layer can be controlled by the flow rate of the doping gas in the source and carrier gases. The upper limit is regulated by the dopant solubility in the epitaxial layer.
When an n type Al.sub.x Ga.sub.1-x As (hereinafter referred to as "n-Al.sub.x Ga.sub.1-x As") epitaxial layer is grown on the substrate and an undoped or a p type Al.sub.y Ga.sub.1-y As (hereinafter referred to as "p-Al.sub.y Ga.sub.1-y As") epitaxial layer is grown thereon using H.sub.2 Se as n type dopant in the apparatus of FIG. 8, the supply of H.sub.2 Se into reaction tube 2 is halted when the growth of the n-Al.sub.x Ga.sub.1-x As epitaxy layer concludes as shown in FIG. 9. Ideally, the introduction of Se into the epitaxial layer is completely ended at this stage, but, practically, H.sub.2 Se molecules which have attached to the internal wall of reaction tube or gas inlet tube remain. This results in the carrier concentration profile shown in FIG. 9. Such a phenomenon is called as "H.sub.2 Se memory effect".
The laser doubleheterojunction (hereinafter referred to as "DH") of FIG. 10 can be produced using H.sub.2 Se as an n type dopant as will be described below.
In FIG. 10, reference numeral 10 designates an n-GaAs substrate. An n-Al.sub.x Ga.sub.1-x As first cladding layer 11 is grown on the substrate 10. A p-Al.sub.y Ga.sub.1-y As active layer 12 is grown on the first cladding layer 11. A p-Al.sub.x Ga.sub.1-x As second cladding layer 13 is grown on the active layer 12. A p-GaAs contact layer 14 is grown on the second cladding layer 13. A pn junction is produced between the n-Al.sub.x Ga.sub.1-x As first cladding layer 11 and the p-Al.sub.y Ga.sub.1-y As active layer 12. When this device is used for a CD laser, the n-Al.sub.x Ga.sub.1-x As first cladding layer 11 with x=0.5 having a film thickness of about 2.5 microns, the p-Al.sub.y Ga.sub.1-y As active layer 12 with y=0.15 having a film thickness of about 800 angstroms, the p-Al.sub.x Ga.sub.1-x As second cladding layer 13 with x=0.5 having a film thickness of about 1.2 microns, and the p-GaAs contact layer 14 having a film thickness of about 0.5 microns, are preferably employed.
FIG. 11 shows a growth program of the MOCVD method which for obtaining the DH shown in FIG. 10 in the prior art semiconductor thin film crystal growth apparatus shown in FIG. 8. As shown in FIG. 11, when the growth of n-Al.sub.x Ga.sub.1-x As first cladding layer 11 concludes, the supply of H.sub.2 Se is halted and the supply of DMZn is started. When such a growth program is used, the position of pn junction 15' often deviates from the presupposed position 15 toward the p-Al.sub.x Ga.sub.1-x As second cladding layer 13 due to the above-described H.sub.2 Se memory effect, as shown in FIG. 12. The width of positional deviation of the pn junction varies depending on the state of the apparatus, more concretely, the state of the internal walls of the reaction tube or of the gas inlet tube.
In order to prevent such a positional deviation of the pn junction, the supply of H.sub.2 Se is halted before Se reaches the design position of the pn junction as shown in FIG. 14(a). Then, the doping of Se concludes at the pn junction design position in view of the H.sub.2 Se memory effect, as shown in FIG. 14(b). In this method, however, when the wafer is at high temperature in a later thermal annealing process, Zn as the p type dopant for producing the p-Al.sub.y Ga.sub.1-y As active layer 12 diffuses to the n-Al.sub.x Ga.sub.1-x As first cladding layer 11 provided therebelow, and the actual pn junction position is deviated from the design position toward the first cladding layer 11. Particularly, since Zn is diffused with a steeper profile than the other dopant, control of the pn junction position is difficult. In case of a CD laser, because the active layer generally is rather thin, i.e., about 500 to 1000 angstroms thick, even when the pn junction position is slightly deviated from the design position (it is called as a "remote junction"), the laser characteristics deteriorate to a great extent.
In order to prevent remote junctions due to the H.sub.2 Se memory effect, a dopant source having a low memory effect is required. SiH.sub.4 is well known as such a dopant source. However, as shown in FIG. 13, SiH.sub.4 has the disadvantage of low doping efficiency at the usual MOCVD growth temperature of about 600.degree. to 750.degree. C. This is because the doping efficiency of SiH.sub.4 is mainly regulated by its thermal decomposition speed. Since a dopant concentration of about 1.times.10.sup.17 to 1.times.10.sup.19 cm .sup.-3 is required to produce a semiconductor laser, it is difficult to produce an epitaxial layer for a semiconductor laser using SiH.sub.4 as a dopant gas in MOCVD method.
Problems in the prior art semiconductor thin film crystal growth by MOCVD method and the apparatus therefor are summarized as follows.
(1) In a case where H.sub.2 Se is used as a dopant gas, when the supply of dopant gas into the reaction tube is halted, the doping of Se into epitaxy layer does not immediately stop. Therefore, a steep doping profile cannot be obtained. Furthermore, as is easily presumed from this fact, accurate control of the pn junction position is difficult.
(2) When SiH.sub.4 having a low memory effect is used as the dopant gas, the doping efficiency is regulated by the thermal decomposition speed of SiH.sub.4, and a high dopant concentration cannot be achieved at a practical crystal growth temperature.